How to Test a Car Battery with a Multimeter
Dealing with a car that won’t start can be frustrating and stressful. Often, the first suspect is the battery. But how can you tell if the battery is dead, needs a jump, or requires replacement? A simple multimeter test can quickly determine if your battery is properly charged.
Here’s a step-by-step guide to testing your car battery with a multimeter.
What Voltage Should a Car Battery Have?
A fully charged, healthy car battery should read approximately 12.6 volts when tested at rest.
- Test the battery after the car has been sitting for at least an hour to get an accurate resting voltage.
- If you’ve recently driven the car, the battery may display a higher voltage due to the charging system’s activity. For the best results, test the battery in the morning after leaving the car parked overnight.
Step 1: Locate and Access the Battery
- Find the battery: Most car batteries are in the engine bay on one side of the engine. If it’s not visible, check your vehicle’s owner manual for the location. Some cars may have the battery in the trunk or under a seat.
- Remove covers: Modern vehicles often have a plastic cover over the battery. Unclip or remove it, and ensure the metal terminals are accessible. The positive terminal may have a red cover, which can also be snapped or lifted off.
- Exercise caution: Avoid letting any metal objects touch both terminals simultaneously, as this can cause a short circuit.
Step 2: Set Up Your Multimeter
A multimeter is a tool used to measure electrical properties such as voltage, current, and resistance.
- Turn the multimeter to DC voltage: Look for the "V" symbol with a straight line above it.
- Choose the appropriate range: If your multimeter has ranges like 2V, 20V, 200V, etc., select 20V, as it’s the closest to the expected 12V reading.
- Connect the test leads:
- Insert the black lead into the multimeter port labeled “COM” (common/ground).
- Insert the red lead into the port labeled “V” (voltage).
Step 3: Test the Battery
- Connect the leads:
- Touch the red lead to the positive (+) terminal of the battery.
- Touch the black lead to the negative (-) terminal or another grounded part of the vehicle, such as an unpainted metal surface.
- Read the voltage:
- A healthy battery will show approximately 12.6 volts when fully charged.
- If the voltage is 12.4 volts, the battery is about 75% charged.
- A reading below 12.2 volts indicates the battery is under 50% charged and may need charging or replacement.
Additional Tips for Accurate Testing
- If your battery voltage is 10 volts or lower, it’s likely dead or has a cell issue.
- If you get inconsistent or fluctuating readings, check for corrosion or loose connections at the terminals and clean them before testing again.
Why Use a Multimeter?
Multimeters are affordable and versatile tools. They range in price from $20 to several hundred dollars and are invaluable for diagnosing electrical issues beyond just testing car batteries.
By regularly checking your battery’s health, you can avoid unexpected breakdowns and ensure your vehicle stays reliable.
To measure battery load, conduct three voltage tests with the multimeter. Start the first test by making sure the vehicle is turned off. Multimeters have two probes: red and black.
The red probe is for contact with the positive terminal and the black probe is for contact with the negative terminal. Connect the multimeter’s red probe with the positive battery terminal and the black probe with the negative terminal.
The multimeter display should show a reading of 12.2 to 12.6 volts indicating a full charge. This voltage range means the battery is in good condition for starting the vehicle.
If the measured reading is less than 12.2 volts, the battery’s resting voltage is weak, which means it most likely needs to be charged or replaced.
Once the resting voltage has been determined, it is time to get a reading on the crank cycle.
This is the moment that the vehicle is turned on and the battery is under the most draw because of the higher amount of energy needed to drive the starter motor. To get this reading, a second person will be needed for the purpose of starting the ignition.
As soon as the car is turned on, the voltage reading will drop for a quick moment, but should not fall below 10 volts. If it falls below 10 volts, it means the battery does not have sufficient turnover strength and is prone to failure.
Again, in this case, recharge or replacement of the battery may be necessary.
Immediately following the crank cycle, the vehicle will begin to idle and maintain a steady draw from the battery. With the motor running, the multimeter rating should stay in the 14 to 14.5 volt range.
Dropping below 14 means either the battery is weak and unreliable for sustained vehicle operation or the alternator is failing. The alternator’s job is to generate energy to feed the electrical system and charge the battery while the vehicle is running.
Related Post: How to Charge a Car Battery?
Understanding Voltage Readings
Interpret voltmeter readings as:
- 12.4V to 12.7V — Fully charged and good condition battery.
- 12.2V to 12.3V — Low end of normal range. Retest under load.
- 12.0V — Borderline reading. Charge fully and retest.
- 11.8V or lower — Discharged battery. Will need replacement soon.
- 10V or lower — Extremely discharged. Battery likely faulty and should be replaced.
Voltage under 12V nearly always indicates a bad battery needing replacement.
How to check your alternator with a multimeter?
An alternator produces electricity and charges the battery. Not only that, but while you’re driving it also takes over and delivers power to the car’s electrical systems.
So with the engine running (and being mindful about moving parts) conduct the same battery test as above with your multimeter. A healthy charging system should give a reading of between 13.8V and 14.4V at regular idle speed.
Anywhere outside that range and your car’s either under, or overcharging – both of which will shorten the battery life and require further investigation. For more on the charging system in your car, see When Good Alternators Go Bad (and Why).
How to find dead cells in a car battery
First things first, knowing a battery cell is dead is of no more use to you than knowing that the battery won’t hold a 12.6 volt charge, because you can’t fix it. So, a test of the resting voltage is just as effective a diagnosis.
A battery usually won’t go bad all at once; instead, some of the cells that make up the battery will go dead. An engine that’s slow to turn over when starting is often a sign of dead battery cells.
Most new car batteries are of the sealed ‘maintenance free’ variety, but some older batteries allow you to access the individual cells. The cells will either have individual caps, or a plastic cover that clips over all, or several of the cells. The cells contain a mix of water and sulfuric acid, so make sure you don’t get any liquid on your skin.
To test each cell, you need a hydrometer-style battery tester that measures the specific gravity of the battery acid. Based on the number of balls floating, you can tell whether the cell is dead or not.
Pro Tip: Before you disconnect your battery make sure you know the anti-theft code for your stereo – you may find it won’t work when you reconnect it otherwise!
Related Post: How To Test an Alternator with A Multimeter?
Tips for Accurate Voltage Testing
Follow these tips for reliable results testing battery voltage:
- Maintain secure lead connections — clean terminals thoroughly first
- Make sure ignition and all accessories are OFF while testing resting voltage
- Retest alternately at negative and positive terminals for complete context
- Repeat tests under various loads to simulate real world conditions
- Test consistently over time and track changes to identify failing batteries
With practice, you will become adept at diagnosing battery issues quickly through voltage and load testing.
How Much Voltage Does A Car Battery Need?
A battery needs the bulk of its voltage in order to function properly. While some people think that a battery has to get down to zero volts before it stops working, the reality is that a car battery can’t dip too far below 12 volts before it’s unable to perform its duties and turn your vehicle on.
Here’s a car battery voltage chart that correlates a battery’s voltage to its life, to help display how many volts are really needed to keep your car running happily.
| Voltage | State of the Battery’s Charge |
| 12.6 or higher | 100% |
| 12.5 | 90% |
| 12.42 | 80% |
| 12.32 | 70% |
| 12.2 | 60% |
| 12.06 | 50% |
| 11.9 | 40% |
| 11.75 | 30% |
| 11.58 | 20% |
| 11.31 | 10% |
| 10.5 or lower | Dead |
Related Post: How Much Should A Car Battery Cost?
FAQs.
What is a Multimeter?
A multimeter measures the voltage in your car battery, letting you know how much power is currently stored in the battery.
How Do I Know if My Car Battery is Bad?
The most common symptoms that come with a bad battery include:
1. Battery light illuminated on the dashboard
2. Engine cranks slowly when starting
3. Vehicle requires frequent jump starts
4. Clicking when you turn the engine
5. Lights are dim
6. Car won’t start
How Long Should a Car Battery Last?
Most car batteries are warranted to last for four years, but only around 30% of batteries made today make it to that point.
What Else Could Be Wrong with My Car Battery?
Another common problem with your car battery is corrosion at the terminal connections. You can remove the corrosion with sandpaper, but make sure to wear gloves and safety goggles!
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